This invention relates to the cooling of electrical transformers, and more particularly to transformers with disc or flat coil windings having provision for improved natural circulation of the liquid coolant.
Liquid-cooled medium and power transformers of the type having disc coil windings mounted about a magnetic core structure are commonly either force-cooled by pumping the insulating oil or other coolant through the windings, or are cooled by natural circulation of the coolant upwardly through the windings by the free convection mechanism. In these liquid-cooled transformers, a pair of concentric cylindrical duct walls are mounted within and surrounding the disc winding, thereby defining inner and outer axial ducts for coolant flow in the vertical direction at each side of the coils. In force-cooled equipment, prior practice has been to use complete barriers in the vertical cooling ducts at alternate vertically spaced locations inside and outside the disc coils to thereby circulate the pumped coolant in a zig-zag path through the winding. Since there is some coolant flow through the horizontal ducts between adjacent coils, this arrangement has good heat transfer characteristics.
As a variation in forced-oil cooled transformers, it is disclosed in Japanese Utility Model Application No. SHO 43-2020 published under Utility Model No. SHO-46-15364 that the alternate complete barrier rings or inserts placed at intervals of several coils to establish a zig-zag flow path can be supplemented by graduated partial barrier rings or inserts in each section to provide more uniform velocity of flow between the disc coils. The horizontal length of the partial barrier ring increases with vertical height in each section to thereby achieve increased resistance to flow in the several horizontal ducts and result in an equal balance of flow resistances. When used in conjunction with the complete barriers, however, the effect of adding graduated partial barriers alternately in the vertical cooling ducts is to increase the total flow resistance. The Japanese patent configurations are illustrated and discussed in application Ser. No. 527,549 filed Mar. 27, 1974, now U.S. Pat. No. 3,902,146 by R. Muralidharan, entitled "Transformer with Improved Forced Liquid Cooled Disc Winding".
Transformer disc coils cooled by natural circulation only must be considerably derated in order to avoid excessive winding temperature rise. Ordinarily, the vertical cooling ducts are provided without flow barriers to minimize flow pressure drop and thus maximize vertical coolant flow past the winding. This results in poor heat transfer due to the absence of or excessively limited coolant flow horizontally between the individual disc coils. The use of alternate complete flow barriers in the vertical cooling ducts, such as is employed in force cooled systems, is not desirable in natural circulation arrangements since full barriers result in too low a coolant flow and consequent excessive temperature raise.